Embossing cylinder with interchangeable sleeve and with system for locking the sleeve at the ends and embossing machine comprising said cylinder

ABSTRACT

The rotating cylinder for processing a web material comprises an essentially rigid cylindrical core ( 13 ), on which is fitted an interchangeable sleeve ( 45 ) of elastically expandable material whose rigidity is less than that of the material of the core; the sleeve is axially and angularly fixable to the core and is provided on its outer surface with a relief pattern. For each end of the  3  cylinder, there is provided at least one corresponding mechanical locking member which can be fixed to the corresponding end of the core of the cylinder and which interacts with the corresponding end of the interchangeable sleeve to lock the interchangeable sleeve angularly with respect to the core of the cylinder.

TECHNICAL FIELD

The present invention relates to a rotating cylinder for processing aweb material, and particularly but not exclusively to an embossingcylinder for embossing sheet materials, such as tissue paper or thelike. More particularly, the invention relates to a rotating cylinder ofthe type comprising an essentially rigid cylindrical central core, onwhich is fitted an interchangeable sleeve made from elasticallyexpandable material having a lower rigidity than the material of thecore.

The invention also relates to a device comprising at least one rotatingcylinder of the aforesaid type, for example an embossing machine or aprinting machine.

The invention also relates to an interchangeable sleeve for a rotatingcylinder, particularly an embossing cylinder or a printing cylinder ofthe aforesaid type.

PRIOR ART

In the paper converting industry, and in other industries in whichcontinuous web materials are processed, various processes and operationsare carried out on these materials by means of rotating cylinders whichact on the web material. Of these operations, printing and embossingoperations are particularly important for the purposes of the presentinvention. In the processing of tissue paper for the production of rollsof toilet paper, rolls of paper towels, paper tissues and napkins andthe like, the paper material is usually subjected to a mechanicaloperation of embossing, which deforms and/or breaks the fibers toincrease the absorbency, volume and softness of the material.

Embossing is also used to imprint a particular pattern on the webmaterial.

Examples of embossing devices used for these purposes are described inU.S. Pat. No. 6,053,232, U.S. Pat. No. 5,096,527 and WO-A-9944814.

One of the problems encountered in this particular technology consistsin the necessity of changing the embossing patterns with a certainfrequency, when changing from one production batch to another, in orderto adapt to the different characteristics of the material used, wheredifferent needs and/or market requirements have to be met.

Similar problems are encountered in the printing of web materials,particularly paper materials. In this case also, the patterns have to bereplaced at a certain rate.

Conventionally, an embossing cylinder consists of a single block ofsteel or other hard material capable of withstanding high bendingstresses without excessive deformation. The outer surface of thecylinder is engraved mechanically, chemically, by laser treatment, or byother methods, to form the embossing pattern, which essentially consistsof a combination of points or protuberances of various shapes, whichprocess the web material which passes between the embossing cylinder anda pressure roller, or between two embossing cylinders havingcomplementary patterns.

In this case, the replacement of the embossing pattern requires thereplacement of the whole cylinder. The costs of the cylinder and of theengraving operations are very high. Consequently, if a given number ofinterchangeable embossing patterns is required, it is necessary to storea corresponding number of expensive cylinders, and also to carry outparticularly lengthy manual operations to replace the cylinder, withconsequent loss of production as a result of the machine downtime.

To alleviate this problem, systems for facilitating the replacement ofthe pattern on the embossing cylinder have been investigated.EP-A-0836928 describes an embossing device in which each embossingcylinder consists of an elongated internal cylindrical core on which aninterchangeable sleeve is fitted. This sleeve carries the embossingpattern. The interchangeable sleeve is elastic and can be expandeddiametrically to facilitate its fitting onto the central core of thecylinder and its removal therefrom. The fitting and removal of thesleeve are facilitated by a compressed air system which serves to expandthe interchangeable sleeve to facilitate its removal from the core or tofacilitate its fitting.

This system is also used in printing cylinders for replacing a printingblock formed on an interchangeable sleeve. Printing cylinders of thistype are described in EP-A-0181726, EP-A-0009360 and U.S. Pat. N.4,144,813.

One of the problems arising in this type of solution, and in theembossing industry in particular, consists in the tendency of the sleeveto slip or rotate on the central core of the cylinder. This is due tothe elasticity of the material used, the tangential forces applied, andthe heating to which these mechanical elements are subject duringoperation. This slippage creates problems of wear and, particularly indouble embossing units, a misalignment of the positions of theengravings on the two embossing rollers with respect to each other. Toprevent the angular displacement of the interchangeable sleeve withrespect to the core of the cylinder, studies have been carried out onshape couplings consisting of longitudinal grooves and longitudinal ribson the two components forming the cylinder. However, this solution isnot satisfactory, particularly when the interchangeable sleeve is madefrom glass fiber reinforced plastic. The presence of continuous glassfibers positioned helically in the body of the sleeve makes theformation of grooves or projections unsuitable. The sleeve must have asmooth inner surface.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a cylinder forprocessing a web material in sheet form, and particularly to provide anembossing cylinder, with an interchangeable sleeve, which does not havethe drawbacks of conventional cylinders.

In particular, the object of the present invention is to provide anembossing cylinder with an interchangeable sleeve in which theinterchangeable sleeve can be locked angularly with respect to the coreof the cylinder.

This and other objects and advantages, which the following text willmake clear to those skilled in the art, are essentially achieved with arotating cylinder comprising a substantially rigid axial cylindricalcore on which is fitted an interchangeable sleeve of elasticallyexpandable material whose rigidity is less than that of the material ofthe core, and additionally comprising, at each end of the cylinder, atleast one mechanical locking member which can be fixed to thecorresponding end of the core of the cylinder and which interacts withthe corresponding end of the interchangeable sleeve to lock saidinterchangeable sleeve angularly with respect to the core of theembossing cylinder.

Advantageously, the two mechanical locking members can be fixed to thecorresponding end faces of the core of the cylinder. Thus the wholecylindrical surface of the central core remains free and the axiallength of the interchangeable sleeve can be equal to the axial length ofthe core. Furthermore, the locking is facilitated because the lockingmembers are accessible from the end faces. The locking is advantageouslyachieved by means of screws, although other systems of locking the partstogether are not ruled out.

In a possible embodiment, each of the mechanical locking memberscomprises a bracket which can be fitted onto the end faces of thecylinder and which is provided with at feast one appendage extendingonto the cylindrical surface of the cylinder and engaging with theinterchangeable sleeve.

Advantageously, the interchangeable sleeve can consist of two coaxialcylindrical components connected together, the inner cylindricalcomponent having an inner surface for connection to the surface of saidcore and the outer cylindrical component having said embossing patternon its outer surface. In practice, the inner cylindrical component canbe made from glass fiber reinforced plastic and the outer cylindricalcomponent can be made from an elastomeric material, for example ebonite,although the use of other materials is not ruled out, provided that theouter component has sufficient hardness and that the overall elasticityis such that a slight deformation is permitted at the time of fitting onthe core or removal from the core.

In a possible embodiment, the mechanical sleeve locking bracket can havea tooth which is inserted into a recess formed in the edge of theinterchangeable sleeve. In particular, when the sleeve is formed with anelastomeric outer cylindrical component, it is advantageous to provideat each end of the sleeve an essentially rigid ring, within which isformed one or more recesses for one or more appendages of the lockingbracket. This makes it possible to avoid forming notches, which couldform possible points of initiation of rupture, in the cylindricalcomponents forming the sleeve. Additionally, the ring can be made fromsteel or other hard material which provides sufficient strength in thearea in which the forces applied by the appendage of the locking bracketare concentrated.

In a different embodiment, each of said mechanical locking memberscomprises a conical forcing surface which interacts with a complementaryconical surface portion formed at each end of said interchangeablesleeve.

Further advantageous embodiments of the cylinder according to theinvention are indicated in the attached claims.

The invention also relates to a cylindrical interchangeable sleeve for arotating cylinder for processing a web material, where the cylindercomprises a substantially rigid central core on which saidinterchangeable sleeve is fixed angularly and axially, saidinterchangeable sleeve having a relief pattern on its outer cylindricalsurface, and being characterized in that it comprises, at each end, anelement for fixing to a mechanical locking member for locking saidinterchangeable sleeve with respect to said core.

Further advantageous characteristics of the sleeve according to theinvention are indicated in the attached dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the description andthe attached drawing, which shows a non-restrictive practical example ofthe invention. In the drawing,

FIG. 1 shows schematically an embossing machine in which the presentinvention can be applied;

FIG. 2 shows a side view, in partial longitudinal section, of anembossing cylinder;

FIG. 3 shows in a greatly enlarged longitudinal section one end of theembossing cylinder in a first embodiment;

FIG. 4 shows a view taken on IV-IV as marked in FIG. 3;

FIG. 5 shows a view taken on V-V as marked in FIG. 3; and

FIGS. 6 and 7 show two greatly enlarged details of the two opposite endsof the embossing cylinder in a second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows in a highly schematic way an embossing machine, indicatedin a general way by 1, in which the present invention can be applied.However, it should be understood that the fundamental principles of theinvention are also applicable to the production of embossing cylindersfor use in embossing machines of other types, for example “nested”machines or single embossing devices. The invention can also be used forthe production of cylinders for processing web materials, in order tocarry out operations other than embossing, for example printing or otherprocesses which may require the rapid replacement of an interchangeablesleeve fitted on the cylinder, wherever problems of angular slippingbetween an interchangeable sleeve and the central core of the cylinderoccur.

The embossing machine 1 is of what is known as the tip-to-tip type, andhas a first embossing cylinder 3 and a second embossing cylinder 5, eachprovided with an interchangeable sleeve. The structure of the embossingcylinders is described in detail below. The embossing pattern,consisting of a plurality of protuberances, is formed on the outersurfaces of the embossing cylinders. The embossing cylinders 3 and 5interact with corresponding pressure rollers 7 and 9. A first ply of webmaterial, indicated by V1, passes between the pressure roller 7 and theembossing cylinder 3, and is embossed there. A second ply, indicated byV2, is embossed between the pressure roller 9 and the embossing cylinder5. The two embossed plies are laminated and joined in the nip betweenthe embossing cylinders 3 and 5 to form the web material N. The joiningis carried out by means of adhesive applied by an adhesive dispenser 11.

The structure of the embossing machine described here in summary form isknown, and will not be illustrated in greater detail in this document.

FIG. 2 shows one of the two embossing cylinders 3 and 5. These can beessentially identical to each other, and therefore only one of them, inparticular the cylinder 3, will be illustrated in greater detail. It hasa cylindrical central core 13 extending along the axis of the embossingcylinder and supported at its ends by bearings 15 and 17 housed inbushes 19 and 21. In the illustrated example, the core 13 is formed bytwo flanged end shanks 13P and 29, which terminate and support acylindrical shell 13M. The two shanks are inserted into the bearings 15and 17.

The bush 21 is mounted directly in a side piece 23 of the embossingmachine, while the bush 19 is inserted into a flange 25 fixed to thesecond side piece 27 of the embossing machine and releasable therefrom.The shank 29 of the cylindrical core 13 extends beyond the bearing 17and on said shank is keyed a pulley 31 which imparts the motion to theembossing cylinder. The shank 29 extends beyond the pulley 31 to form anend 29A which can be engaged by a bracket 33 hinged at 35 to a rod 37which can be moved vertically by an actuator 39 as shown by the arrowf37. FIG. 2 shows the two possible positions of the bracket 33. In thefirst position, shown in solid lines, the bracket 33 is in a verticalconfiguration and engages the end 29A of the shank 29 with a shapedwheel 41. When the bracket 33 is in this position, the actuator 39 isoperated until the bearing contained in the bush 29 is completelyreleased from the weight of the cylinder. At this point, the flange 25can be removed, and the embossing cylinder 3 can be cantileverlyretained by the bearing 17 only, so that the interchangeable sleeve canbe replaced by the methods described below. In the position shown inbroken lines, the bracket 33 is in a horizontal configuration and theembossing cylinder 3 can rotate about its axis indicated by A-A.

An interchangeable sleeve, indicated in a general way by 45, is fittedon the core 13 of the embossing cylinder 3. The external diameter of theinterchangeable sleeve 45 is smaller than the hole which remains free inthe side piece 27 after the flange 25 has been extracted from it. Thus,the sleeve 45 can be extracted from the cylindrical core 13 and replacedwith another by engaging the cylinder 3 with the bracket 33 and removingthe flange 25 (so that the cylinder 3 is cantileverly retained).

The interchangeable sleeve 45 consists of two cylindrical components,which can be seen in particular in the section in FIG. 3, which shows anenlargement of the detail indicated by III of the right-hand end of theembossing cylinder of FIG. 2, with the interchangeable sleeve mountedand locked on the internal cylindrical core 13. The components formingthe interchangeable sleeve 45 are indicated by 47 and 49. The internalcylindrical component 47 is advantageously made from glass fiberreinforced plastic, in other words with a structure consisting of ahelical distribution of glass fibers in a matrix of polymerized resin.On the other hand, the cylindrical component 49 consists of a very hardelastomer, for example ebonite or a synthetic elastomer, whose hardnessis equivalent and which is in any case suitable for the purposes forwhich the interchangeable sleeve is intended. The number 49A indicatesthe protuberances which are formed on the outer surface of theinterchangeable sleeve, and which therefore consist of the materialforming the outer cylindrical component 49 of said sleeve.

To facilitate the fitting of the sleeve 45 on the central core 13 andits removal therefrom, apertures are provided in the central core 13 sothat the interior 13A of the core 13 communicates with the outer surfaceof the cylindrical shell 13M of the core, in other words with thesurface on which the interchangeable sleeve 45 slides. In FIGS. 2 and 3,the numbers 51 indicate in a general way connecting ducts between theinside and the outside of the core 13 of the cylinder 3. When aninterchangeable sleeve 45 is to be removed from the core 13, thecylinder 3 is first engaged by means of the bracket 33 and the flange 25is removed, after which a connection 53 (FIG. 2) is used to pressurizethe inside 13A of the core 13 and to expand the sleeve 45 by means ofthis pressure. The elasticity of the interchangeable sleeve 45 issufficient to reduce the pressure exerted by the sleeve on the outersurface of the core 13, and thus the extraction of the sleeve 45 isfacilitated. The air flowing between the inner surface of the sleeve 45and the outer surface of the core 13 also acts as a lubricant,facilitating the sliding of the two components with respect to eachother. The fitting of a new interchangeable sleeve with a differentembossing pattern is facilitated in a similar way. The compressed airinside the sleeve 13 is introduced by means of a duct connected to asource of compressed air (not shown) which is connected to a connection53 formed in the shank 29.

The elasticity of the interchangeable sleeve 45 is sufficient to locksaid sleeve axially and angularly on the core 13 except in the presenceof differential thermal expansion of these two components and/or hightangential forces. However, when the embossing device is operating, thedissipation of heat caused by the high pressures between the embossingcylinder 3 and the corresponding pressure roller 7 causes an expansionof the interchangeable sleeve 45 by an amount sufficient to cause aslight angular displacement of said sleeve with respect to the core 13.Since the embossing cylinders 3 and 5 are kept in synchronization withreference to the rotary movement of the cores 13, the slippage betweenthe core and the interchangeable sleeve 45 fitted on it ultimatelycauses a misalignment of the embossing patterns between the twocylinders.

In order to overcome this drawback, in the embodiment shown in FIGS. 3,4 and 5 an end-fitted angular locking system is provided at each end ofthe embossing cylinder 3. The two systems are essentially identical, andtherefore only the right-hand end (FIG. 2) of the embossing cylinder isshown in the drawing. As shown in the drawing, at least one bracket 61is fixed on the end face of the core 13 of the embossing cylinder 3, bymeans of screw members 63 which are engaged in holes formed in thethickness of the cylindrical shell 13M of the cylindrical core 13. Aspacer 65 is placed between the end face 13F of the cylindrical shell13M of the core 13 and the bracket 61. This prevents the axial crushingof the sleeve between the two opposite brackets 61.

In the illustrated example, the bracket 61 extends only over a fractionof the circular extension of the end face of the core 13, but there isno reason why the bracket 61 should not have an annular shape, andtherefore an extension of 360°.

The bracket 61 has a tooth 61A parallel to the axis A-A of the embossingcylinder 3. The tooth 61A is inserted into an edge notch or recess 67formed in a terminal ring 69 integral with the inner cylindricalcomponent 47 of the interchangeable sleeve 45. The terminal ring 69 isadvantageously made from a rigid material, for example steel. It ishoused within the overall thickness of the interchangeable sleeve 45,since the axial length of the outer cylindrical component 49 is smallerthan that of the cylindrical component 47. The difference between thelengths of the two components is approximately equal to twice the axialextension of the ring 69, since a terminal ring is located on theopposite end (not illustrated) of the interchangeable sleeve 45.

The internal diameter of the inner cylindrical component 47 at theposition of the rigid ring 69 is slightly greater than that of the restof the component, to facilitate the fitting of the sleeve 45 in spite ofthe lack of radial expandability of the sleeve at the position of thering 69.

Thus the interchangeable sleeve 45 can be angularly locked with respectto the core by locking at least one bracket 61 on each of the two endsof the inner core 13. It is also possible to provide a plurality of endbrackets 61 on each end, and/or brackets having more than one tooth 61Aeach, to obtain a better distribution of stresses.

FIGS. 6 and 7 show an enlargement of the two opposite ends of theembossing cylinder 3 in a different embodiment. Identical numbersindicate identical or corresponding parts in the two embodiments.

In this configuration, the brackets provided at the two ends of theembossing cylinder 3 are indicated by 61 and 62 respectively, and theirshapes are slightly different from each other, for reasons which areexplained below.

The right-hand end (with reference to the configuration in FIG. 2) ofthe core 13 of the embossing cylinder 3 has an annular shoulder 13S onwhich the interchangeable sleeve 45 bears, while the opposite end has noshoulder (FIG. 7), thus enabling the interchangeable sleeve to be fittedand removed.

At each of its two ends, the interchangeable sleeve 45 has an end areawith a conical surface, indicated by 45B, on its outer surface. Theconical surfaces 45B are tapered towards the corresponding edges of thesleeve. Each of the brackets 61 and 62 has a corresponding conicalsurface, indicated by 61A and 62A respectively, whose shape iscomplementary to that of the surfaces 45B. The conical surfaces 61A and61B can have an extension of 360°, thus covering the whole edge of thesleeve. Alternatively, they can have a smaller extension and take theform of appendages distributed suitably along the circumferentialextension of the cylinder. Thus, when each bracket 61 and 62 isfrontally pressed against the corresponding end face 13F of the core 13,the interchangeable sleeve 45 is locked angularly as a result of theinteraction of the two conical surfaces 45B and 62A, or 45B and 61A, andthe consequent pressure of the interchangeable sleeve against thecylindrical shell 13M of the core 13.

In the illustrated example, the brackets 61 and 62 have an annularextension. The bracket 61 is shaped in such a way that it can becentered and guided onto the shoulder 13S during the tighteningmovement, while the bracket 62 is shaped to be centered and guided ontothe cylindrical surface of smaller diameter 13T formed on thecorresponding end of the core 13. The two brackets 61 and 62 could havea partial extension instead of a full annular extension. In this casethey will be guided and centered also onto the inner edge of the shell13M.

It is to be understood that the drawing shows only an example providedsolely as a practical demonstration of the invention, and that thisinvention can be varied in its forms and arrangements without departurefrom the scope of the guiding principle of said invention. The presenceof any reference numbers in the attached claims has the sole purpose offacilitating reading with reference to the attached drawings and to thepreceding text, and does not limit the scope of protection of theclaims.

1. A rotating cylinder for processing a web material, comprising an essentially rigid cylindrical core on which is fitted an interchangeable sleeve of elastically expandable material whose rigidity is less than that of material of the core, said sleeve being axially and angularly fixable to said core and being provided on an outer surface thereof with a relief pattern, wherein, at each end of the cylinder, at least one corresponding mechanical locking member is present which can be fixed to the a corresponding end of the core of the cylinder and which interacts with a corresponding end of the interchangeable sleeve to lock said interchangeable sleeve angularly with respect to the core of the cylinder, and wherein each said mechanical locking member comprises a bracket which fits onto an end face of the cylinder and which is provided with at least one appendage extending onto a cylindrical surface of the cylinder and engages the interchangeable sleeve.
 2. Cylinder according to claim wherein each said mechanical locking member can be fixed to the a corresponding end face of the core of the cylinder.
 3. Cylinder according to claim 1, wherein each said mechanical locking member can be fixed to the core of the cylinder by screw means.
 4. Cylinder according to claim 1, wherein said bracket has a form of a section of a circular ring.
 5. Cylinder according to claim 1, wherein said bracket has an annular extension.
 6. Cylinder according to claim 1, wherein said interchangeable sleeve is formed from two coaxial cylindrical components fixed rigidly to each other, an inner cylindrical component thereof having an inner surface for connection to a surface of said core and an outer cylindrical component having said embossing pattern on an outer surface thereof.
 7. Cylinder according to claim 6, wherein said inner cylindrical component is made from glass fiber reinforced plastic and said outer cylindrical component is made from an elastomeric material.
 8. Cylinder according to claim 1, wherein said bracket is inserted into a recess formed in an edge of the interchangeable sleeve.
 9. Cylinder according to claim 8, wherein said interchangeable sleeve comprises, at each end, an essentially rigid ring, within which is formed one or more recesses for one or more of said at least one appendage.
 10. Cylinder according to claim 6, wherein the inner cylindrical component has an axial length which is greater than that of the outer cylindrical component and projects from the outer cylindrical component at both ends, and wherein an essentially rigid ring is placed around each edge of said inner cylindrical component.
 11. cylinder according to claim 10, wherein said inner cylindrical component has an end portion with an increased internal diameter at the a position of each o said essentially rigid ring.
 12. Cylinder according to claim 1, wherein each said mechanical locking member comprises a conical forcing surface which interacts with a complementary conical surface portion formed at each end of said interchangeable sleeve.
 13. Cylinder according to claim 12, wherein said conical forcing surface of each said mechanical locking member is formed on said appendage.
 14. Cylinder according to claim 12, wherein said locking member comprises a centering and guide surface interacting with a corresponding centering and guide surface integral with the core of the cylinder.
 15. Cylinder according to claim 12, wherein said core of the cylinder comprises at one end of said core a projecting annular stop on which said sleeve bears.
 16. Cylinder according to claim 15, wherein said projecting annular stop forms the a centering and guide surface integral with the core of the cylinder.
 17. Cylinder according to claim 1, wherein said core of the cylinder comprises channels for introducing compressed air between the outer surface of said core and the an inner surface of said interchangeable sleeve.
 18. Cylinder according to claim 1, wherein said cylindrical core is an embossing cylinder, said relief pattern on the outer surface of the interchangeable sleeve being formed by a plurality of embossing protuberances.
 19. An embossing machine comprising at least one pressure roller and at least one embossing cylinder, wherein said embossing cylinder is made according to one of claims 1 to
 18. 20. A printing machine for printing a web material, comprising at least one impression cylinder and a plate cylinder, wherein said plate cylinder is made according to one of claims 1 to
 18. 21. A cylindrical interchangeable embossing sleeve for a rotating embossing cylinder for processing a web material, which can be locked to an essentially rigid cylindrical core, said interchangeable sleeve comprising an embossing pattern on an outer cylindrical surface thereof, a fixing element at each end of said sleeve for fixing to a mechanical locking member for locking said interchangeable sleeve with respect to said core; the interchangeable sleeve further including two coaxial cylindrical components fixed rigidly to each other, an inner cylindrical component having an inner surface for connection to a surface of said core and an outer cylindrical component having said embossing pattern on an outer surface thereof; and said fixing element is formed on said outer cylindrical component.
 22. Interchangeable sleeve according to claim 21, wherein said inner cylindrical component is made from glass fiber reinforced plastic and said outer cylindrical component is made from an elastomeric material.
 23. Interchangeable sleeve according to claim 21 or 22, wherein said fixing element comprises at least one recess along each terminal edge.
 24. Interchangeable sleeve according to claim 23, wherein said fixing element comprises two rigid end rings, in each of which at least one recess is formed.
 25. Interchangeable sleeve according to claim 21, wherein the inner cylindrical component has an axial length which is greater than that of the outer cylindrical component, and projects from the outer cylindrical component at both ends, and wherein one of said essentially rigid ring is placed around each edge of said inner cylindrical component.
 26. Interchangeable sleeve according to claim 25, wherein said inner cylindrical component has an end portion with a slightly increased internal diameter at a position each of said essentially rigid rings.
 27. Interchangeable sleeve according to claim 21, wherein said fixing element comprises a conical forcing surface which interacts with a complementary conical surface portion formed on a mechanical locking member. 